Differential microphones with selectable beampatterns (i.e., response patterns) have been in existence now for more than 50 years. For example, one of the first such microphones was the Western Electric 639B unidirectional microphone. The 639B was introduced in the early 1940's and had a six-position switch to select a desired first-order pattern. Unidirectional differential microphones are commonly used in broadcast and public address applications since their inherent directivity is useful in reducing reverberation and noise pickup, as well as feedback in public address systems. Unidirectional microphones are also used extensively in stereo recording applications where two directional microphones are aimed in different directions (typically 90 degrees apart) for the left and right stereo signals.
Configurations of four-element cardioid microphone arrays arranged in a planar square arrangement and at the apices of a tetrahedron for general steering of differential beams have also been proposed and used in the past. (See, e.g., U. S. Pat. No. 3,824,342, issued on Jul. 16, 1974 to R. M. Christensen et al., and U. S. Pat. No. 4,042,779 issued on Aug. 16, 1977 to P. G. Craven et al.) However, none of these systems provide a fully steerable and variable beampattern at a reasonable cost. In particular, none of these prior art microphone arrays make use of (inexpensive) omnidirectional pressure-sensitive microphones in combination with a simple processor (e.g., a DSP), thereby enabling, at a modest cost, precise control of the beam-forming and steering of multiple first-order microphone beams.